Seven speed MTB style shifters are available just about anywhere on line. Amazon for one, but there are hundreds of sources selling them. Personal taste here, but I can't stand the twist grip shifters. Assuming you have Shimano derailleurs (big jump as you did not mention either way), the $13 Tourney trigger shifters are my favorite, and I put them on just about every MTB from that era, including the three in my keeper fleet right now.

For $13, you get the shifters, new cables and new housings. Tough to beat IMHO.

OK, another assumption, new shifter not working right (Shimano with Shimano?), then you have another issue, either a set up problem, cable housing, or bent derailleur hanger. All are easy fixes. Did you reuse old housing? Park tool site has good instructions on setting up shifters.

I am not a fan of the Sunrace shifters, the ones I have seen were really cheap.